1. Technical Field
The present invention is directed to lithographic processes for device fabrication and, in particular, to such processes that utilize reflective masks with multilayer films for patterning radiation.
2. Art Background
In lithographic processes for device fabrication, radiation is typically projected onto a patterned mask and the radiation transmitted through the mask is further transmitted onto an energy sensitive material formed on a substrate. Transmitting the radiation through a patterned mask patterns the radiation itself and an image of the pattern is introduced into the energy sensitive material when the energy sensitive resist material is exposed to the patterned radiation. The image is then developed in the energy sensitive resist material and transferred into the underlying substrate. An integrated circuit device is fabricated using a series of such exposures to pattern different layers formed on a semiconductor substrate.
Patterned masks do present certain problems as well as advantages. The advantage of a patterned mask is speed. In one brief exposure, a pattern is introduced over a substantial surface area of the substrate. This is considerably faster than a direct-write exposure technique, where a beam of radiation (most typically an electron beam) is used to "write" the pattern in the energy sensitive material. In a direct-write technique, an area of the energy-sensitive material only as large as the area of the beam is exposed at a given moment.
The disadvantage of patterned masks is that they must be extremely precise. One defect in a mask, if undetected, can result in the loss of hundreds and even thousands of integrated circuit chips. Therefore, masks must not only be manufactured precisely, they must be carefully inspected so that any mask defects are detected before the mask is used in a manufacturing process.
Mask inspection techniques, like the masks themselves, are adapted to be compatible with the specific lithographic process in which the mask is used. The materials of the mask are selected to be compatible with the exposing radiation. For example, in lithographic processes that use optical or ultraviolet radiation as the exposing radiation, glass masks (i.e. a metal patterned film formed over a glass substrate that is transparent to the exposing radiation) are typically used. In a projection lithography tool that uses electrons, the mask consists of higher density regions of material that scatter electrons incident thereon (the blocking regions) and much lower density regions that do not scatter electrons incident thereon (the non-blocking regions).
As design rules (i.e. the size of the features in the integrated circuit pattern) decrease from 0.5 .mu.m to 0.35 .mu.m to 0.25 .mu.m to 0.18 .mu.m, etc., the wavelength of the exposing radiation also decreases because the smallest feature a projection lens can resolve is proportional to the wavelength. For fabricating integrated circuit devices with 0.1 .mu.m size features, exposing radiation in the wavelength range referred to as extreme ultraviolet (EUV) has been proposed.
Masks that are compatible with EUV radiation are somewhat different than masks that are compatible with longer wavelengths of light because the glass substrates that are typically used for optical lithography are not transparent to EUV radiation (i.e. radiation with a wavelength in the range of about five nm to about fifty nm).
In order to overcome this problem, EUV masks are substrates (typically fused silica or silicon) that are coated with a multilayer film that reflects the EUV radiation. Over this film is a patterned layer which absorbs EUV radiation. Thus, the radiation reflected from the mask surface is patterned. The nature of the EUV masks makes it especially difficult to inspect for the presence of certain types of defects.
For example, some defects on the patterned material are readily observed optically. This is because the EUV mask is typically four times larger than the pattern that is formed on the device substrate. Thus a 0.15 .mu.m feature on the device substrate would be formed using a corresponding feature on the mask that was four times larger (i.e. 0.6 .mu.m). A feature of such size is readily observed using optical inspection techniques. The presence of particulate contamination, i.e. dirt particles, which can also be a source of defects, is also detected by optical examination.
However, some defects are not susceptible to detection using optical inspection because they do not cause an observable change in the visible wavelength radiation reflected therefrom. Examples of such defects include defects which cause reduced reflectivity from the film in the region of the defect or defects which shift the phase of the EUV radiation reflected from the film in the region of the defect. The nature of these defects is that they often cannot be detected using optical inspection techniques because, at the optical inspection wavelengths, these types of defects are invisible. However, these defects will affect the EUV radiation that falls within the reflective bandpass of the multilayer. Consequently, if such defects are not detected, the resulting mask will produce a defective pattern on the device wafer. Since the pattern on the device wafer is four times smaller than the pattern on the mask, the defect in the pattern could escape optical detection, but manifest itself in the defective operation of the resulting device.
Therefore, it is desired to inspect the mask substrate for all defects before the patterned multilayer film is formed on the mask, and further desired to inspect the patterned mask for defects before it is used to form a pattern on a device wafer. The sooner that a defect is detected, the fewer the adverse consequences that will result from the defect. However, in order to detect defects that are not susceptible to detection by optical techniques, the inspection must be made using radiation in the range of wavelengths that are reflected by the multilayer film (i.e. the EUV wavelengths).
One method for inspection of EUV masks at an EUV wavelength is described in Nguyen, K. B., et al., "Imaging of EUV Lithographic Masks with Programmed Substrate Defects" OSA Proceedings on Extreme Ultraviolet Lithography, Vol. 23, pp. 193-203 (1994). In the technique described in Nguyen et al., incident EUV radiation is directed onto the mask, and the reflected radiation is imaged onto a film of energy sensitive material. The resulting image in the energy sensitive material is developed, and the pattern is inspected for defects attributable to the mask. This technique is limited because certain defects in the mask (e.g. a slight reduction in the reflectivity of the substrate or a defect that causes only a slight phase change in the reflected radiation) are not likely to be detected. Furthermore, the technique is time consuming since multiple separate exposures are required to transfer an image of the entire mask into the energy sensitive resist material.
Accordingly, a technique for inspecting EUV masks that is both fast and capable of detecting even very subtle defects that are not detected by the currently available techniques is desired.